Prosthetic heart valves are widely used for the replacement of natural valves as well as ventricular assist devices and artificial hearts. Valves can cause blood damage which may lead to complications such as hemolysis and thromboembolism. Hemodynamic stresses imposed on the blood elements as they pass through the valve are believed by many to play a major role in blood damage. The hemodynamic stresses are generally thought to derive from turbulent flow induced by the valves, but more recently, the formation and collapse of cavitation bubbles near mechanical heart valves at closure have been implicated in both blood and valve material damage. Most studies of heart valve fluid dynamics to date have focused on the turbulent stresses generated during forward flow through the open valve using two component laser-Doppler anemometry (LDA). During the current grant period the applicant developed 3-component LDA techniques with high resolution (1 msec time windows) and applied them to the study of mechanical heart valve fluid mechanics during the regurgitant closure phase and sustained leakage flow phase of tilting disk and bi-leaflet mechanical heart valves under operating conditions characteristic of both natural and artificial hearts. In the next grant period, the applicants proposed to continue studies which test the hypothesis that fluid stresses induced by valve closure and sustained leakage flow are dominant blood damaging mechanisms in mechanical heart valves. To improve their understanding of the complex flow structures created by heart valves at closure the applicants proposed to apply modern techniques of particle image velocimetry (PIV) to complement the three-dimensional LDA methods developed during the current grant. Specific aims of the proposed research are: 1. To obtain point measurements of the instantaneous velocity field and associated Reynolds stresses near four different heart valve types in the mitral position using high resolution, three- dimensional LDA. The effects of maximum ventricular dP/dt, maximum ventricular pressure and valve disk mechanical properties on turbulent stresses at closure and in sustained leakage flow will be determined; and 2. To develop PIV for application near mechanical heart valves at closure. Using PIV the applicants proposed to determine the vortex and squeeze flow structures which induce high fluid stresses, vaporous cavitation and stable gas bubble formation at valve closure; and